In recent years, water-absorbent resins have been widely used in the fields of hygienic materials such as sanitary articles and disposable diapers.
For water-absorbent resins as described above, crosslinked products of partially neutralized polymers of acrylic acid are preferred because they have many advantages, including the followings: they have excellent water-absorption performance; their raw materials such as acrylic acid has easy industrial availability, and therefore they can be produced with stable quality and low cost; and they show no shortcomings such as in which decomposition is likely to occur.
In the fields of hygienic materials such as sanitary articles and disposable diapers, such a water-absorbent resin is used to form an absorbent material, and by the means that the absorbent material is held between a liquid permeable sheet (top sheet) through which a liquid can pass and a liquid impermeable sheet (back sheet) through which a liquid cannot permeate, it is possible to form an absorbent article.
An absorbent material formed with a water-absorbent resin is generally formed with the water-absorbent resin and a hydrophilic fiber. An absorbent material is formed as, for example, a mixed dispersion member obtained by mixing a water-absorbent resin and a hydrophilic fiber such that they have a uniform composition, a sandwich structure member in which a water-absorbent resin is sandwiched between layered hydrophilic fibers or a wrapped member in which a water-absorbent resin and an hydrophilic fiber is wrapped with tissue. Here, it is important to form an absorbent material in which a water-absorbent resin is uniformly dispersed in order to manufacture an absorbent article having an excellent quality.
In the manufacturing of the absorbent article having the absorbent material described above on an industrial scale, the water-absorbent resin may be exposed to the atmosphere for a long period of time such as by an unexpected stop of manufacturing facilities. Here, since the water-absorbent resin is hygroscopic, a part or the whole thereof may be aggregated. The water-absorbent resin aggregated as described above is significantly lowered in flowability, and thereby experiences troubles such as the occurrence of blocking within the piping of manufacturing facilities and the formation of a bridge within a powder hopper. Furthermore when the absorbent material described above is prepared, the aggregated water-absorbent resin may hinder its uniform dispersion.
In recent years, since an absorbent article and an absorbent material used for it tend to be decreased in thickness, there a tendency that the proportion of a bulky hydrophilic fiber and the like is reduced and to increase the proportion of the water-absorbent resin. Therefore, the properties of the water-absorbent resin suitable for the manufacturing facilities of the absorbent article tend to more significantly affect the ease of manufacturing of the absorbent material and the absorbent article using it.
In order to enhance the moisture absorption resistance of the water-absorbent resin described above, a technology is proposed in which surface treatment is performed on the particles of the water-absorbent resin with a surfactant such as a silicone-based surfactant or polyurethane having a specific functional group (see Patent Documents 1 and 2).
However, the water-absorbent resins obtained in these methods do not necessarily satisfy the problems described above, and there are still improvements to be made.
Moreover, conventional water-absorbent resins may be, depending on each resin individually, colored pale yellow or dark yellow in the state of a product. When as described above, the proportion of the water-absorbent resin is increased, and thus the absorbent material is decreased in thickness, if the water-absorbent resin is colored, the cleanliness of appearance of an absorbent article using that resin is lowered, with the result that the product value tends to be lowered.